1. Field of the Invention
The present invention concerns the reconstruction of images of a subject using raw data acquired from a positron emission tomography (PET) scan or a single-photon emission computed tomography (SPECT) scan.
2. Description of the Prior Art
Medical or veterinary imaging, for example cardiac imaging, often requires a set of scans to be acquired in rapid succession, for example showing rest and stress states for myocardial perfusion studies.
In methods such as PET or SPECT scan imaging, muscular tissue such as the heart it not itself directly imaged. Rather, a tracer is introduced into a patient's bloodstream, and the imaging process provides images of the location of the tracer. The imaging effect of the tracer diminishes with time, at a rate which is characterized by a half-life. Depending on the half-life of the tracer, second and consecutive scans could detect remaining residual activity from previous doses of tracer, which would affect the contrast and image quality and bias the quantification assessment of the scan.
To overcome this problem, consecutive scans are often acquired with delays in between, e.g. half an hour or more for 13NH3 scan, or respective scans are performed on different days such as for 99mTc SPECT imaging. This means longer waiting time for the patients and reduced scanner throughput.
Another known way to reduce the effect of tracer residual from earlier scans is to use a low dose for a first scan and a much higher dose for a second scan, for example with a dose ratio of 1:5. The residual activity of the first tracer in the second scan, for example about half an hour later, is low compared with the high injected dose; therefore the bias of image quality is kept to certain controllable degree. This is proposed for 18Flurpiridaz scans which use the radionuclide 18F and has a half-life of 110 minutes. This half-life is too long for implementing a delayed scan protocol for same day scanning. However, as this method involves much higher dose for the second scan, the image quality is different for the first and second scans due to dose difference, and the bias introduced by the residual activity potentially reduces the accuracy of the assessment.
Existing methods of removing the deleterious effects of residual tracer activity are often based on subtraction of reconstructed images. An example of such a conventional process will be discussed with reference to example images shown in FIGS. 1A-1C, which each show three views of a single PET image.
An early frame image is shown in FIG. 1A, as acquired before the tracer is injected for a further scan, so that it contains only the residual activity from a previous scan.
FIG. 1B shows corresponding late frame images, taken after a further tracer has been introduced.
The early frame image of FIG. 1A is subtracted from the late frame image acquired after tracer injection as in FIG. 1B. An example result image is shown in FIG. 1C, which is intended to represent the later frame image of FIG. 1B with the residual effects of the earlier tracer removed.
A problem with such subtraction method is that image noise is effectively amplified, potentially leaving large negative values which are shown as dark speckles in the subtracted image. Examples of such speckles may be seen in FIG. 1C. Sometimes other image artefacts could be introduced, due to motion or noise.
Another conventional residual activity correction method based on time activity curves (TAC) where the tracer uptakes over time (referred as TAC) in ROIs are derived. The first time point of a TAC represents the residual activity, and is subtracted from all the time points of the TAC. The corrected TACs are then used in the kinetic model fitting for the quantification of physiological effects. This method does not produce a residual corrected anatomical image such as FIG. 1C for visualization.
Such methods are described, for example, in X-B Pan, E Alexanderson, L Le Meunier, J Declerck, Residual activity correction for computing myocardial blood flow from dynamic 13NH3 studies, J. NUCL. Med. MEETING ABSTRACTS, May 2011; 52: 2103 and S. G. Nekolla; S. Reder; A. Saraste; T. Higuchi; G. Dzewas; A. Preissel; M. Huisman; T. Poethko; T. Schuster; M. Yu; S. Robinson; D. Casebier; J. Henke; H. J. Wester; M. Schwaiger, Evaluation of the Novel Myocardial Perfusion Positron-Emission Tomography Tracer 18F-BMS-747158-02: Comparison to 13N-Ammonia and Validation With Microspheres in a Pig Model, Circulation, 2009; 119: 2333-2342.
Assumptions made in these methods include:                listmode acquisition starts a short period, such as 10 seconds, before the later introduction of tracer.        uptake of the residual activity has reached equilibrium, and can be modelled by radioactive decay only.        